Especially in the machine tool field, but not exclusively, it is an objective to provide a handle for manually turning a shaft, perhaps to move a tool saddle. It is conventional practice for this shaft also to be powered by another source such as an electrical or hydraulic motor. The problem which results is that unless the handle is specifically disengaged, it will turn when the shaft is driven by the other source. Then a person standing nearby may be struck or caught by the rotary handle, and be injured.
It is an object of this invention to provide a safety handle which can bi-directionally rotate a shaft, engaging the shaft while so doing. But when the safety handle is released, it will automatically be disengaged from the shaft and rotate freely relative to it. Bystanders cannot then be struck by it, because it will not be turning, or if it is, it will be freely turning and will immediately stop if it encounters any resistance.
Because this is a safety device, it is necessary for its disengagement to be automatic, without requiring anything more than for the operator to let go of it. Because it should also be a convenient device to use, it should require no special exertion, or any manipulation not ordinarily expected to be exerted to turn a handle.
According to this invention, the twisting torque exerted on any handle to turn it will also be sufficient to cause driving engagement with the shaft, and the release of this torque will result in prompt disengagement.